Aluminous metal article



Unite 3,098,724 ALUMINOUS METAL ARTICLE William A. Anderson, Verona, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania N Drawing. Filed July 8, 1960, Ser. No. 41,503 Claims. (Cl. 29195) This invention relates to bright aluminous metal articles, and more particularly, articles which have a highly specular finish combined with a pleasing cast or tinge which modifies the natural aluminum color.

The ability of aluminum and aluminum base alloys to receive and retain attractive decorative finishes has been amply demonstrated in service over a long period of time in both indoor and outdoor exposures. A wide variety of these finishes has been employed ranging from the natural appearance to colored finishes. Among the finishes are those which produce diifuse light reflection and those which are bright and give a specular reflection of light. For the latter purpose it has been a common practice to brighten the surface of the article by mechanical, chemical or electrochemical means, or a combination of them, and apply a protective transparent coating, the color being the natural one supplied by the metal itself. To achieve the highest specular reflectivity it has been found that very high purity aluminum should be employed without modification by any alloy additions.

Although the natural color of aluminum and aluminum base alloys is satisfactory for many purposes there are places where that color is not considered to be harmonious with adjacent structures, for example, those having a chrome plated finish, and some modification of the color is therefore desirable. In addition to the matter of altering the color there is often a need for a higher strength and hardness than provided by high purity aluminum. This need is particularly evident Where abrasion and scratching are encountered as in the decorative trim on automotive vehicles and household appliances.

The well known anodic oxidation process has been frequently used to improve the resistance of aluminum to abrasion. However, the nature of the oxide film varies with the electrolyte, the composition of and the conditions under which the coating is formed and hence not all films are equally suitable to resist abrasion. Also, some electrolytes, as well as some brightening solutions, tend to etch the metal surface and produce a microscopic roughening thereby giving the treated surface a whitish or cloudy appearance which detracts from the brilliance of the finish. This condition is referred to as bloom. The presence and degree of bloom is physically determined by light reflection measurements with an instrument such as a goniophotometer and is defined as the ratio of light diffusely reflected at 47 to that specularly reflected at 45 when the incident light beam is at an angle of 45 to the reflecting surface. This is generally expressed in terms of percent of the specular reflectance. In addition to bloom the oxide film may be colored by certain alloying elements or impurities depending on the amount present in the alloy. To gain the highest specular reflectivity and image clarity, the finish must be substantially free from bloom and discoloration.

It is an object of this invention to provide an aluminous metal article having an aluminum base alloy surface which when suitably brightened and given an anodic treatment acquires a high brilliance and a slight yellowish tinge with high image clarity.

Another object is to produce an aluminous metal article having an aluminum base alloy surface which is specularly reflective surface and has a yellowish cast but which also has a higher strength than high purity aluminum.

A particular object is to provide a 'Wrought aluminum base alloy article having a specularly reflective surface with a yellowish cast which closely resembles a chrome plated finish that is adapted to serve as decorative trimming on automotive vehicles and household appliances.

It has been found that these and other objects and advantages can be obtained by employing either as the sole structural component or as the surface component of a composite article an aluminum base alloy consisting of 0.4 to 2% magnesium, 0.05 to 0.15% copper, 0.05 to 0.15% chromium, from a trace up to 0.05% titanium and as impurities up to 0.15 iron, up to 0.15% silicon and up to 0.05% manganese, the balance being aluminum. It is to be understood that other impurities may be present, namely, those normally associated with aluminum, but when in the amounts usually found in aluminum they have no effect upon the color and strength of the foregoing alloy. Since manganese is only present as an impurity and the maximum permitted is only 0.05%, the alloy is regarded as being substantially free from that element. It is important that the proportions of the added elements and maximum amount of iron and silicon be observed if the desired results are to be obtamed. If greater amounts of magnesium are present, there is danger of producing surface defects arising from insoluble magnesium-containing compounds. In .the alloys containing magnesium, the copper and chromium contents must be controlled in order to achieve the desired color cast and brilliance with the substantial absence of bloom. It has been discovered that both copper and chromium must be employed in combination with magnesium to gain the desired result, and preferably, that they be present in the ratio of from 1 to 3 parts of chromium to 1 part of copper. The chromium and copper appear to supplement each other, neither one alone being capable of producing the desired combination of cast of color and freedom from bloom. If the maximum limits on iron and silicon are exceeded, the finish becomes dull and streaks are apt to appear. With the proper brightening and anodic treatments the alloy article acquires an appearance which is very similar to that of a chrome plated article.

In referring to the substantial absence of bloom it is to be understood that bloom is not necessarily completely absent, but may be present in such small amounts as to be insignificant. When the bloom value is less than 0.5%, the article is considered to be substantially free from bloom.

To secure the best results the following ranges and impurity limits should be observed: 0.6 to 1.0% magnesium, 0.05 to 0.15 copper, 0.05 to 0.15 chromium and a maximum of 0.02% titanium, 0.01% manganese, 0.06% silicon and 0.08% iron. In the annealed temper the alloy in sheet form has a typical tensile strength of 17,000 p.s.i., a yield strength of 6500 p.s.i. and an elongation of 25%. In the temper resulting from col-d rolling with a reduction of in thickness, it has a typical tensile strength of 29,000 p.s.i., a yield strength of 27,000 and an elongation of 5%. In comparison high purity aluminum (99.99%) sheet when annealed has a tensile strength of 9000 p.s.i., a yield strength of 2000 p.s.i. and an elongation of 55%. When cold rolled with a reduction of 75%, the metal has a tensile strength of 16,500 p.s.i., a yield strength of 15,000 and an elongation of 6.5%.

The yellowish cast imparted to the oxide coated article is evident when such an article is compared to a similarly coated article made from high purity aluminum. The existence and extent of the yellow coloration can be determined in a photoelectric tristimulus colorimeter, such as described in the National Bureau of Standards Circu Patented July 23, 1963' The yellowness is deterwhere A is amber, B is blue and G is a green filter, and the values so obtained are multiplied by 100 thus giving a percent figure. These values should fall Within the range of 1.80 to 6.0. Lower values show no benefit and higher values reach into the area of a definite color rather than a tinge or cast. An article composed of an alloy of aluminum and 0.06% chromium under the same conditions had a reading of only 0.69.

To obtain the desired finish the article should be given a brightening treatment of either the chemical or electrochemical type, for example, those disclosed in United States Patents 2,108,603, 2,096,309, 2,650,157 and 2,729,- 551. These treatments produce a bright specularly reflective surface of uniform quality if the alloy has a homogeneous structure.

Although the structure of the wrought alloy article may be homogeneous as a result of the normal ingot casting and metal working practices, it is often desirable to insure such homogeneity by treating the ingot before it is hot worked. Such a treatment may consist of soaking the ingot at a temperature of 1100 to 1150" F. for a period of 4 to 16 hours, cooling to room temperature and reheating to the hot working temperature. Alternatively, the ingot may be directly cooled to the hot working temperature and worked. In either case those constituents which have been dissolved by the soaking treatment appear to remain in solution in the Wrought product.

The article which has been brightened, as described above, is then given an anodic treatment in a conventional acid electrolyte which is capable of developing a clear transparent oxide film. Suitable electrolytes for this purpose include sulphuric acid, and mixtures of sulphuric acid and other acids such as chromic, phosphoric, oxalic and sulpho salicyclic. The oxide film must also have a minimum thickness in order to obtain the desired color effect, that thickness being 0.1 mil. On the other hand, the film should not exceed about 0.6 mil in thickness. To develop the desired film, a 15% sulphuric acid solution can be employed and the treatment extended over a period of 5 to 40 minutes at a current density of 12 amperes per square foot, or under conditions which achieve an equivalent number of ampere-minutes per square foot.

The article coated in the foregoing manner possesses a slightly yellowish cast or tinge which becomes evident when the article is compared to high purity aluminum which has also received an anodic treatment.

Reference has been made to the worked condition of the article. This may be effected by any of the well known metal working processes, such as rolling, extrusion, forging, drawing, and the like. Generally, the ingot is hot worked, and if the article is not of the desired thickness, it is then cold worked. Cold working does not destroy or adversely afiect the desired coloration. To improve the forming characteristics, it may be desirable to subject the cold wonked article to a thermal treatment, known as recovery, to relieve some of the work hardening strains.

Where the aluminum-magnesium-chromium-copper alloy is employed as a surface component or cladding on another aluminum base alloy component, the two components may be metallurgically bonded by known methods. To insure a complete and uniform bond, it may be desirable to use an aluminum inter-layer or its metallurgical equivalent. This method is also known in the art and forms no part of the process herein described. If the alloy is used as a cladding, the base or core member may consist of any one of the conventional structural aluminum base alloys, for example, those of the alumimum-manganese, aluminum-magnesium, aluminum-cop- 2. per and aluminum-magnesium-silicon types. In view of the fact that the article may be a composite body composed of the aluminum-magnesium-copper-chrominum alloy described [above and one or more other aluminum base alloys, or that it may consist entirely of the aforesaid alloy, it is referred to herein as being an aluminous meta-l article.

My invention is illustrated in the following examples.

Specimens of sheet of three alloys in the cold rolled (75% reduction) temper and having the compositions shown below, aluminum being the balance in each case, were polished by buffing, cleaned, chemically brightened in a solution of the type disclosed in United States Patent 2,650,157 and anodically coated for 15 minutes in a 15% sulfuric acid electrolyte at a temperature of 8488 with a current density of 12 amperes per square foot. Alloy 1 was included in the tests to show the effect of chromium in the absence of copper. The yellowness values of the anodized specimens were determined with a commercial photoelectric tristimulus colorimeter and the aver-age value calculated. The bloom values of the anodized specimens were measured with a goniophotometer and the average determined.

Percent composition by weight Percent Alloy Percent yellowbloom ness Cu Fe Si Mg Cr Ti Alloys 2 and 3 having compositions within the limits defined herein above, had significantly lesser amounts of bloom and higher degrees of yellowness than alloy No. 1. There was no visible evidence of a hue or cast when Alloy 1 was compared with the other alloy specimens. Also, in comparison, high purity aluminum (99.99%) sheet when cold rolled and finished in a same manner had percent bloom and yellowness values of 0.78 and 0.00 respectively. The comparison is particularly significant for high purity aluminum has usually been considered to acquire the highest mirror-like finish of aluminous metal articles.

Having thus described my invention and certain embodiments thereof, 1 claim:

1. An aluminous metal article having a specularly reflective surface with an anodically produced film thereon substantially free from bloom and having a percent yellowness value of 1.8 to 6 as determined by a photoelectric tristimulus colorimeter, the oxide film being at least 0.1 mil in thickness, the metallic surface of said article underlying the oxide film consisting of an alloy composed of aluminum, 0.4- to 2.0% magnesium, 0.05 to 0.15% copper and 0.05 to 0.15% chromium, the impurities present including up to 0.15 iron, up to 0.15 silicon and up to 0.05% manganese, said alloy surface extending over at least one entire face of said article.

2. An article according to claim 1 wherein the alloy contains from a trace up to 0.02% titanium.

3. An article composed of an aluminum-magnesiumcopperchromium alloy having a specularly reflective surface with an anodically produced film thereon substantially free irom bloom and having a percent yellowness value of 1.8 to 6 as determined by a photoelectric tristimulus colorimeter, the oxide film being at least 0.1 mil in thickness, said alloy being composed of aluminum, 0.4 to 2% magnesium, 0.05 to 0.15% copper and 0.05 to 0.15% chromium, the impurities present including up to 0.15 iron, up to 0.15 silicon and up to 0.05% manganese.

4. An article according to claim 3 wherein the alloy also contains from'a trace up to 0.02% titanium.

I 5. An article according to claim 3 wherein the magnesium content is 0.6 to 1.0% and the maximum iron content is 0.08%, the maximum silicon content is 0.06% and the maximum manganese content is 0.01%.

'6. An article according to claim 3 wherein it has been cold worked and subjected to a recovery treatment.

7. An anticle according to claim 3 wherein the ratio of chromium to copper is 1 to 3 parts of chromium to 1 part of copper.

8. A composite aluminous metal article consisting of at least two components, one of which is a base and the other a cladding extending over at least one entire face of the base, said aintiele having a specularly reflective surface with an anodically produced film on said cladding substantially free irom bloom and having a percent yellowness value of 1.8 to 6 as determined by a photoelectric tristimulus colorimeter, and oxide film being at least 0.1 mil in thickness, the cladding component of said article consisting of an alloy composed of aluminum, 0.4 to 2.0% magnesium, 0.05 to 0.15% copper and 0.05 to References Cited in the file of this patent UNITED STATES PATENTS Hall Nov. 18, 1930 Fromson June 21, 1960 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,098,724 July 23, 1963 William A. Anderson It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 5, line IS for "and" read the column 6, line 11, for "0.8%" read 0.08%

Signed and sealed this 7th day of January 1964.

ggesxf' EDWIN L. REYNOLDS ERNEST W. SWIDER Attesting Officer Ac ting Commissioner of Patents 

1. AN ALUMINOUS METAL ARTICLE HAVING A SPECULARLY REFLECTIVE SURFACE WITH AN ANODICALLY PRODUCED FILM THEREON SUBSTANTIALLY FREE FROM BLOOM AND HAVING A PERCENT YELLOWNESS VALUE OF 1.8 TO 6 AS DETERMINED BY A PHOTOELECTRIC TRISTIMULUS COLORIMETER, THE OXIDE FILM BEING AT LEAST 0.1 MIL IN THICKNESS, THE METALLIC SURFACE OF SAID ARTICLE UNDERLYING THE OXIDE FILM CONSISTING OF AN ALLOY COMPOSED OF ALUMINUM, 0.4 TO 2.0% MAGNESIUM, 0.05 TO 0.15% COPPER AND 0.05 TO 0.15% CHROMIUM, THE IMPURITIES PRESENT INCLUDING UP TO 0.15% IRON, UP TO 0.15% SILICON AND UP TO 0.05% MANGANESE, SAID ALLOY SURFACE EXTENDING OVER AT LEAST ONE ENTIRE SURFACE OF SAID ARTICLE. 